Thermal pyrolytic graphite laminates with vias

ABSTRACT

A thermally conductive laminate comprising a first substrate, a second substrate, and a performance layer disposed between the first substrate and the second substrate. The performance layer comprising thermal pyrolytic graphite (TPG) and vias. The TPG board surface and the vias may be at least partially filled with a material comprising at least one of thermally conductive epoxy, soldering metal/alloy or brazing metal/alloy. In addition, the thermally conductive laminate may not contain a framing structure surrounding the performance layer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of and claims priority to U.S. patent application Ser. No. 11/555,681 filed Nov. 1, 2006, which is a continuation-in-part of U.S. application Ser. No. 10/761,567, filed Jan. 21, 2004, now U.S. Pat. No. 7,220,485, issued May 22, 2007, which claims the priority benefit of U.S. Patent Application No. 60/743,998 filed Mar. 30, 2006.

FIELD OF INVENTION

The present invention relates to a thermal management assembly including but not limited to a heat transfer device, which may also be referred to as a heat spreader, that can be used for transferring heat away from a heat source, e.g., to a heat sink; an assembly having the heat spreader in contact with the heat source, e.g., between the heat source and the heat sink; and a heat sink for dissipating the heat. The invention also relates to methods of manufacturing a thermal management assembly.

BACKGROUND

Many forms of thermal management exist today all of which depend upon the principles of conduction, convection or radiation to move heat. Good thermal conductivity is required to permit heat transfer away from high density electronic components and devices such as integrated circuits. High thermal conductivity materials are conventionally used in heat transfer devices to dissipate heat from semiconductor circuits and systems. Elemental metals are not satisfactory for the semiconductor circuit systems in use today. Heat transfer devices with high thermal conductivity materials may also be utilized in aerospace and military applications. This has led to the use of high conductivity heat transfer devices formed from composites or laminations of different materials fabricated into various structural assemblies which will possess the desired high thermal conductivity, strength and other needed properties.

The heat transfer device is physically connected between a heat source, which generates considerable waste heat, and a heat sink. In many cases, however, the heat source and heat sink are not in close proximity and one or both may not be easily accessed for interconnecting a heat transfer device. In these situations the heat transfer device needs to be malleable and flexible. Currently available heat transfer devices which have very high thermal conductivities relative to the thermal conductivity of elemental metals are not readily usable in these situations.

SUMMARY

In one aspect, the present invention provides a thermally conductive laminate comprising a first substrate, a second substrate, and a performance layer disposed between the first substrate and the second substrate. The performance layer comprises thermal pyrolytic graphite (TPG) having a plurality of vias. The vias occupy from about 0.1% to about 20% volume, which may also be referred to as the via loading density, of the thermal pyrolytic graphite. The TPG board surface and the vias may be at least partially filled with a bonding material.

Applicants have found that providing the holes or vias in the graphite at this loading density provides a heat transfer laminate exhibiting both excellent mechanical and conductive properties. For example a laminate with a thermal pyrolytic graphite layer having a via loading density of from about 0.1% to about 20% may exhibit a sufficient bond strength (e.g. greater than 40 psi) overcome the thermal stress at 300° C. but also exhibit excellent in-plane thermal conductivity (e.g. greater than 1,000 W/m-K).

In another aspect, the present invention provides a heat transfer device comprising a thermally conductive laminate comprising a first substrate; a second substrate; and a performance layer disposed between said first substrate and said second substrate, said performance layer comprising thermal pyrolytic graphite having a plurality of vias disposed therein; wherein said heat transfer device is free of a framing structure surrounding said performance layer. Applicants have found that the laminate with the filled vias provide sufficient structural stability that additional framing surrounding the graphite layer is not necessary. The absence of a frame structure allows large heat transfer assemblies to be provided as thermal laminates that may be pieced or linked together, e.g., by tiling two or more laminates, to form a layer laminate structure. Additionally, the absence of a frame allows end users to cut a laminate to a desired shape as may be required for a particular end use.

This invention also addresses the issues of cost to remove heat in space constrained areas where thermal management by conduction requires a material that can be easily configured to provide a low-density, flexible, thin cross-section for the movement and redistribution of heat loads from heat sensitive electronic components or systems to areas where heat may be dissipated.

DESCRIPTION OF THE DRAWINGS

Objects and advantages together with the operation of the invention may be better understood by reference to the following detailed description taken in connection with the following illustration, wherein:

FIG. 1 illustrates a perspective view of a thermally conductive laminate;

FIG. 2 illustrates a cross-sectional view of the laminate of FIG. 1;

FIG. 3 illustrates a process for forming a thermally conductive laminate in accordance with one embodiment of the invention;

FIG. 4 illustrates thermal conductivity of a thermally conductive laminate as a function of via loading density in accordance with an embodiment of this invention;

FIG. 5 illustrates measured bonding strength of a thermally conductive laminate as a function of via loading density in accordance with an embodiment of this invention;

FIG. 6 illustrates in-plane thermal conductivities of various materials;

FIG. 7 illustrates a comparison between measured bonding strength of thermal pyrolytic graphite and thermal pyrolytic graphite laminates with optimized via loading density and selected thermally conductive epoxy in accordance with an embodiment of this invention.

DETAILED DESCRIPTION

Reference will now be made in detail to exemplary embodiments of the present invention, examples of which are illustrated in the accompanying drawing. It is to be understood that other embodiments may be utilized and structural and functional changes may be made without departing from the respective scope of the invention. As such, the following description is presented by way of illustration only and should not limit in any way the various alternatives and modifications that may be made to the illustrated embodiments and still be within the spirit and scope of the invention.

As used herein, the term “graphite layer” refers to a single cleaving of pyrolytic graphite (PG) comprising at least one graphene layer of nanometer thickness. Also as used herein, the term “cleave” or “cleaving” refers to the process of peeling, removing, or extracting from, or separating a sheet of graphite to obtain at least an ultra-thin layer of graphite, comprising at least one single graphene layer of nanometer thickness. The “sheet” of graphite comprises a plurality of graphene layers.

The term “heat sink” may be used interchangeably with “heat dissipator” and that the term may be in the singular or plural form, indicating one or multiple items may be present, referring to an element which not only collects the heat, but also performs the dissipating function.

As used herein the term “heat spreader” or “heat transfer laminate” may be used interchangeably to refer to a device that is in contact with the source of heat and the heat sink.

Also as used herein, the term “thermal pyrolytic graphite” (“TPG”) may be used interchangeably with “highly oriented pyrolytic graphite” (“HOPG”), or compression annealed pyrolytic graphite (“CAPG”), referring to graphite materials consisting of crystallites of considerable size, the crystallites being highly aligned or oriented with respect to each other and having well ordered carbon layers or a high degree of preferred crystallite orientation, with an in- plane (a-b direction) thermal conductivity greater than 1,000 W/m-K. In one embodiment, the TPG has an in-plane thermal conductivity greater than 1,500 W/m-K

As used herein, the term “graphene” or “graphene film” denotes the atom-thick carbon sheets or layers that stacks up to form “cleavable” layers (or mica-like cleavings) in graphite.

Referring to FIGS. 1 and 2, a heat transfer device 10 comprises a first substrate 12, a second substrate 14, and a performance layer 16 disposed between substrates 12 and 14. The performance layer 16 comprises a thermally conductive material 18 having a high thermal conductivity. The thermally conductive material 18 includes a plurality of vias 20.

The thermally conductive material 18 may be selected from any material having a high thermal conductivity including pyrolytic graphite, thermal pyrolytic graphite, compression annealed pyrolytic graphite, thermal pyrolytic graphite, highly ordered pyrolytic graphite, pyrolytic graphite, and the like. The in-plane thermal conductivity of the high thermal conductivity core material 18 should be greater than 200 W/mK and desirably greater than 500 W/mk for each of the pyrolytic graphite materials. Also as used herein, the term “thermal pyrolytic graphite” (“TPG”) encompasses materials such as highly oriented pyrolytic graphite (“HOPG”), or compression annealed pyrolytic graphite (“CAPG”). In one embodiment, thermal pyrolytic graphite may also refer to graphite materials consisting of crystallites of considerable size, the crystallites being highly aligned or oriented with respect to each other and having well ordered carbon layers or a high degree of preferred crystallite orientation, with an in-plane (a-b direction) thermal conductivity greater than 1,000 W/m-K. In one embodiment, the TPG has an in-plane thermal conductivity greater than 1,500 W/m-K.

Graphites possess anisotropic structures and thus exhibit or possess many properties that are highly directional e.g. thermal and electrical conductivity and fluid diffusion. Graphites are made up of layer planes of hexagonal arrays or networks of carbon atoms. These layer planes of hexagonally arranged carbon atoms are substantially flat and are oriented or ordered so as to be substantially parallel and equidistant to one another. The substantially flat, parallel equidistant sheets or layers of carbon atoms, usually referred to as graphene layers or basal planes, are linked or bonded together and groups thereof are arranged in crystallites. The superposed layers or laminate of carbon atoms in graphite are joined together by weak van der Waals forces.

It will be appreciated that the size and thickness of the thermally conductive material is not particularly limited and may be chosen as desired for a particular purpose or intended use. In one embodiment, the thermally conductive material may be provided as a graphite sheet having a thickness of from about 1 mm to about 5 mm. In another embodiment, the thermally conductive material may be provided as a “graphite layer,” which refers to a single cleaving of a pyrolytic or thermally pyrolytic graphite comprising at least one graphene layer of micrometer or nanometer thickness. Cleaving of graphite to obtain micrometer thick graphite layers and/or ultra-thin nanometer thicknesses is described in U.S. patent application Ser. No. 11/555,681, which is incorporated herein by reference in its entirety.

The thermally conductive material 18 comprises a plurality of vias 20. In one embodiment, the vias occupy from about 0.1 to about 20 volume percent of the thermally conductive material 18. The percent volume of the thermally conductive materially that is occupied by the vias may also be referred to as the via loading density. In another embodiment, the via loading density may be from about 0.01% to about 40%. Applicants have found that providing a thermally conductive material having a via loading density of from about 0.1 to about 20 percent provides a laminate that exhibits both excellent mechanical strength and thermal conductivity. For example, a laminate having a via loading density of from about 0.1 to about 20 percent may exhibit sufficient bonding strength (e.g., greater than 40 psi) to overcome thermal stress at 300° C., and excellent in-plane thermal conductivity (e.g., greater than 1,000 W/mK.

In one embodiment, holes or vias are predrilled into TPG boards at a desired size and spacing to produce optimized results. Thermally conductive epoxy, brazing material, or any other similar material may be applied to the TPG board surface and may be used to fill the via holes either partially or completely. The loading density of the vias may range from less than 0.01% area of occupation to approximately 40% area of occupation. In another embodiment the via loading density may be from about 0.1% to about 20%. In one embodiment, the spacing of the vias may range from about 0.5 to about 125 mm to reach optimal desired results. In another embodiment, the spacing of the vias may range from about 1 to about 25 mm. The TPG board may then be laminated between at least two metal foils. The metal foils may be individually comprised of, but not limited to, copper, aluminum, tungsten, molybdenum, nickel, iron, tin, silver, gold alloys of two or more thereof

In one embodiment and prior to coating, holes or vias with sizes between 0.1 to 5 mm in diameter and spacing between 1 to 25 mm apart are drilled through the ultra-thin graphite layer using methods known in the art including Electro Discharge Machining (EDM), Electro Discharge Grinding (EDG), laser, and plasma. In another embodiment, slits are fabricated in the ultra-thin graphite strip prior to treatments.

In yet another embodiment, louvers, slits or vias are formed or perforated in the graphite layer by any of EDM, EDG, laser, plasma, or other methods known in the art. In one embodiment, vias are formed in the graphite layer so that a diffusion bond can be formed via the plurality of via with a resin coating on both sides of the graphite layer. In one embodiment, the vias may be anywhere from 0.1-5 mm in diameter and placed between 1-25 mm apart to optimize thermal and mechanical performance.

The vias may be filled with a material to provide the laminate with structural support. The vias may be filled with a bonding material such as an adhesive material, a soldering metal or metal alloy, or a brazing metal or metal alloy. Suitable adhesive materials include, for example, inorganic and organic adhesives. An exemplary adhesive material is an epoxy. In one embodiment, the bonding material exhibits thermal conductivity properties, e.g., a thermally conductive epoxy.

The first and second substrates (12, 14) may be formed from a metal material and in one embodiment is provided as a metal foil. The metal may be selected as desired for a particular purpose or intended use. The metal may be chosen, for example, from copper, aluminum, tungsten, molybdenum, nickel, iron, tin, silver, gold or alloys of two or more thereof. The first and second substrates may be made from the same or different metal materials. The thickness of the substrates may be selected as desired for a particular purpose or intended application. In one embodiment, the substrates may each have a thickness of from about 2 microns to about 2 mm.

A thermally conductive laminate may be made by providing a thermally conductive material having a plurality of vias and applying the substrates onto the surface of the thermally conductive material. The vias may be provided in the thermally conductive material by any suitable method including drilling, etching, using liftoff techniques for layer patterning, LIGA (an German acronym for Lithographic, Galvanoformung, and Abformung (Lithography, Electroplating, and Molding)), sacrificial bulk and surface micromachining; and any other known or future developed opening or aperture forming process.

The substrates may be applied to a surface of the thermally conductive material by any suitable method. In one embodiment, as illustrated in FIG. 3, the laminate may be formed by providing metal foils 12 and 14 and laminating them to surfaces 18 a and 18 b, respectively, of thermally conductive material 18. The foils 12 and 14 may be positioned adjacent surfaces 18 a and 18 b, respectively, and the structure passed through rollers 30 a and 30 b to laminate the foils 12 and 14 to the respective surfaces of the thermally conductive material 18. A bonding material 25 may be provided on the surfaces of the thermally conductive material to bond the foils to the thermally conductive material. The bonding material may be provided in discrete areas on the surface of the thermally conductive material or may be applied generally to an entire surface. In one embodiment, the bonding material is at least provided in the vicinity of the vias such that a quantity of bonding material substantially fills the via during the lamination process. It will be appreciated that a curing or activation operation may be required to set or cure the bonding material.

In one embodiment, copper, aluminum, or tinned copper foil tapes backed with a highly conductive pressure-sensitive adhesive are pressed against a pyrolytic graphite substrate and peeled of, for a cleaving of pyrolytic graphite comprising at least one graphene film or layer. In one embodiment, the metal foil has a thickness of about 5.0 to about 25 μm thick, backed with carbon or Parylene, then a layer of highly conductive pressure sensitive adhesives. Metal foil tapes are commercially available from sources including Chomerics and Lebow Company.

In one embodiment employing a thermally conductive epoxy, a curing step may then performed to activate the thermally conductive epoxy, brazing material, or other similar material applied to the TPG board surface and filling the via holes. The cured laminate may then be trimmed to its desired final dimension.

In another embodiment, the substrates may be provided by a coating process such as chemical vapor deposition, physical vapor deposition, plasma vapor deposition, electroplating, electroless plating, dipping, spraying or the like. In the case of ultra-thin heat transfer laminates the substrates may be provided as identified in U.S. patent application Ser. No. 11/339,338 that are incorporated by reference in their entirety.

In one embodiment, a thermally conductive laminate is provided that is free of any structural support member, e.g., a framing structure, encasing the perimeter of the laminate and, in particular, the thermally conductive material. Heretofore, thermally conductive laminates have been provided with such a frame to provide structural support. Applicants have found, however, that such a frame is not needed and that the vias provide the laminate with sufficient structural support. In fact, Applicants have found that the mechanical strength of the laminate increases with a larger via loading density. In an embodiment of a heat transfer device that is free of a structural support/frame member, the via loading density may be provided in the range of from about 0.01% to about 40%. In another embodiment, the via loading density is from about 0.1% to about 20%. In still another embodiment, the via loading density is from about 1 to about 10%. The mechanical strength increases as the via loading density increases, and the thermal conductivity tends to increase as the via loading density decreases. Providing a laminate that is free of any frame structure allows for end users to cut a laminate to a desired shape for a particular purpose or application. Additionally, the size of a laminate is generally limited due to constraints associated with the manufacturing process. By providing a laminate that is free of a frame structure, a plurality of laminates may be bonded together, such as by tiling, to provide a larger laminate structure.

A heat sink design can be a complex task requiring extensive math—finite element analysis, fluid dynamics, etc. In designing heat sinks, various factors are taken into consideration, including thermal resistance, area of the heat sink, the shape of the heat sink, i.e., whether finned or pin design and the height of pins or fins, whether a fan is used and its air flow rate, heat sink material, and maximum temperature to be allowed at die.

Thermal resistance is the critical parameter of heat sink design. Thermal resistance is directly proportional to thickness of the material and inversely proportional to thermal conductivity of the material and surface area of heat flow. The invention relates to an advanced thermal management system with optimized thermal resistance, that may be used to provide a heat sink, or an ultra-thin heat sink comprising a conductive material such a graphite, with thermal conductivity as high as 1,000 W/m-K or more, with a thickness as low as one atomic layer of carbon.

Although the generic term “graphite” may be used herein, an ultra-thin heat sink, depending on the application may employ either pyrolytic graphite (PG) with a typical in-plane thermal conductivity of less than 500 W/m-K, or thermal pyrolytic graphite (TPG) with an in- plane thermal conductivity greater than 600 W/m-K. In one embodiment, the starting feedstock is a graphite sheet commercially available from sources including Panasonic, Momentive Performance Materials, etc.

In one embodiment the TPG board containing vias that may have materials such as thermally conductive epoxy, brazing material, or other similar materials applied to the TPG board surface and at least partially filling at least some of the vias that may be laminated between at least two metal foils is not substantially surrounded by a framing device. In another embodiment there no framing device is used to in connection with the heat sink and/or the thermally conductive laminate.

In a further embodiment, the graphite layer is specifically designed with a number of holes or vias to form a weak mechanical structure, with the filled or coated vias acting to support the structure while minimizing the stress that can be transmitted across the heat sink or thermal spreader. By adjusting the number and location of vias, the thermal conductivity through the TPG and the mechanical integrity of the TPG can be optimized for a particular application, as coating materials (e.g., parylene, metal, etc.) flow into and diffuse across the holes, this creates mechanical vias that cross-link the opposing faces together for improved section modulus. In another embodiment, engineered size and spacing of the vias help mitigate the low z-direction conductivity of TPG, providing enhanced through-the-thickness conductivity in the final product.

In yet another embodiment, the surface of the high thermal conductivity graphite layer is textured or roughened so that the layer can effectively bond and/or adhere to brazing materials, encapsulants or laminating materials.

EXAMPLES

The invention will now be described and may be further understood with respect to the following examples. These examples are intended to be illustrative only and are to be understood as not limiting the invention disclosed herein in any way as to materials, or process parameters, equipment or conditions.

Example 1

A TPG board is provided and via holes are predrilled into the TPG board and then filled with thermally conductive epoxy, thermoplastic, soldering, or brazing material during lamination. Copper laminates having a thickness of 0.1 mm are laminated to opposing surfaces of the TPG board. Several laminates are provided having loading densities of 0%, 1.1%, 4.4%, 7.6%, and 17.6%. The actual and theoretical thermal conductivities and bonding strengths of the boards is determined (see FIGS. 4 and 5). The theoretical conductivities are determined by a series heat flow thermal conductivity tester. As shown in FIG. 4, both the actual and theoretical studies indicate that the thermal conductivity of TPG laminates decreases as the via loading density increases. On the other hand, the bonding strength of TPG laminates rises with the via loading density (FIG. 5).

Example 2

In this Example, the thermal conductivity of a TPG board, a TPG laminate with copper substrates, and a TPG laminate with a copper substrates and a TPG board containing vias with about 1.5 mm diameter and about 6.75 mm spacing with a 4.4% loading density, which is laminated with a thermally conductive epoxy, a copper sheet, and an aluminum sheet are measured. The measured in-plane thermal conductivity of the TPG laminates with vias ranges from 1,200 to 1,400 W/m-K and is compared in FIG. 6 to the thermal conductivity of TPG, a TPG laminate without vias, a copper sheet and an aluminum sheet.

Example 3

Copper laminated and aluminum laminated boards are prepare using a TPG board having a loading density as described in Example 2 and compared to a bare TPG board without vias. As shown in FIG. 7, the copper foil laminated TPG boards exhibited 300% higher bonding strength that the one of bare TPG board, and the aluminum foil laminated of TPG boards exhibited a 600% higher bonding strength. The cooper and aluminum laminated TPG boards did not show any delamination after 60 thermal cycles to 300° C. 

1. A thermally conductive laminate comprising: a first substrate; a second substrate; and a performance layer disposed between said first substrate and said second substrate, said performance layer comprising thermal pyrolytic graphite having a plurality of vias wherein the volume % of said vias in said performance layer rangers from about 0.1% to about 20%.
 2. The thermally conductive laminate of claim 1, wherein said vias are at least partially filled with a bonding material.
 3. The thermally conductive laminate of claim 2, wherein said bonding material is chosen from an organic adhesive, an inorganic adhesive, a soldering metal, a soldering metal alloy, a brazing metal, a brazing metal alloy or a combination of two or more thereof.
 4. The thermally conductive laminate of claim 3, wherein the bonding material comprises a thermally conductive epoxy.
 5. The thermally conductive laminate of claim 1, wherein said thermal pyrolytic graphite surface is coated with a material comprising at least one of a thermally conductive epoxy, a soldering metal, a soldering metal alloy, a brazing metal, or a brazing metal alloy.
 6. The thermally conductive laminate of claim 1, wherein the spacing of said vias ranges from about 0.5 mm to about 125 mm.
 7. The thermally conductive laminate of claim 1, wherein the spacing of said vias ranges from about 1 mm to about 25 mm.
 8. The thermally conductive laminate of claim 1, wherein the first and second substrates are metal foils independently chosen from copper, aluminum, tungsten, molybdenum, nickel, iron, tin, silver, gold, and alloys of two or more thereof
 9. The thermally conductive laminate of claim 1, wherein at least one of said first substrate and said second substrate comprises copper foil.
 10. The thermally conductive laminate of claim 1, wherein at least one of said first substrate and said second substrate comprises aluminum foil.
 11. The thermally conductive laminate of claim 1, wherein the diameter of said vias ranges from about 0.1 mm to about 5 mm.
 12. The thermally conductive laminate of claim 1, wherein said performance layer is surrounded by a framing structure.
 13. A heat transfer device comprising: a thermally conductive laminate comprising: a first substrate; a second substrate; and a performance layer disposed between said first substrate and said second substrate, said performance layer comprising thermal pyrolytic graphite having a plurality of vias disposed therein; wherein said heat transfer device is free of a framing structure surrounding said performance layer.
 14. The thermally conductive laminate of claim 13, wherein said vias are at least partially filled with a bonding material.
 15. The thermally conductive laminate of claim 14, wherein said bonding material is chosen from an organic adhesive, an inorganic adhesive, a soldering metal, a soldering metal alloy, a brazing metal, a brazing metal alloy or a combination of two or more thereof.
 16. The thermally conductive laminate of claim 15, wherein the bonding material comprises a thermally conductive epoxy.
 17. The thermally conductive laminate of claim 13, wherein the volume % of said vias in said performance layer ranges from about 0.01% to about 40%.
 18. The thermally conductive laminate of claim 13, wherein the volume % of said vias in said performance layer ranges from about 0.1% to about 20%.
 19. The thermally conductive laminate of claim 13, wherein the spacing of said vias ranges from about 0.5 mm to about 125 mm.
 20. The thermally conductive laminate of claim 13, wherein the spacing of said vias ranges from about 1 mm to about 25 mm.
 21. The thermally conductive laminate of claim 13, wherein said first and second substrates comprise a material chosen from copper, aluminum, tungsten, molybdenum, nickel, iron, tin, silver, gold, and alloys of two or more thereof.
 22. The thermally conductive laminate of claim 13, wherein at least one of said first substrate and said second substrate comprises copper foil.
 23. The thermally conductive laminate of claim 13, wherein at least one of said first substrate and said second substrate comprises aluminum foil.
 24. The thermally conductive laminate of claim 13, wherein the diameter of said vias ranges from about 0.1 mm to about 5 mm.
 25. A heat transfer structure comprising a plurality of thermally conductive laminates of claim 13 bonded together. 